Method for forming a solid solution alloy crystal

ABSTRACT

A method for forming a solid solution alloy crystal includes forming a solid solution alloy crystal having at least the same diameter as a seed crystal. The seed crystal is exposed to a liquid containing a desired concentration of an alloying element to dissolve a portion of the seed crystal. The solid solution alloy crystal is then formed from the liquid. The method allows a large diameter solid solution alloy crystal to be grown in a reduced time or a larger diameter solid solution alloy crystal to be grown in a known fixed total process time.

BACKGROUND OF THE INVENTION

[0001] This application is a continuation of U.S. application Ser. No.09/336,305 filed Jun. 20, 1999. The entire disclosure of such priorapplication is hereby incorporated by reference.

[0002] The present invention relates generally to a method forsemiconductor wafer manufacturing, and, more particularly, to a methodfor forming a solid solution alloy crystal.

[0003] Silicon is widely used for fabricating semiconductor devices suchas integrated circuits, discrete devices, and sensors. In a typicalintegrated circuit fabrication process, a wafer of crystal material maybe exposed to several processing steps such as doping, etching, orimplanting to form an array of resulting integrated circuits. Theresulting integrated circuits are then individually separated from thewafer and packaged. Typical integrated circuits include severaltransistors interconnected together.

[0004] More recently, the dimensions of semiconductor devices have beenreduced such that more devices are formed per unit area on a singlewafer. Accordingly, various processes have been developed which permitthe forming of larger diameter wafers. This increases the number ofcircuits which can be placed on an individual wafer. Typically, largediameter wafers can be formed from single crystals that are producedusing bulk seeded crystal growing processes to obtain specificcrystallographic orientations.

[0005] One such technique is the floating-zone (“FZ”) process. In thisprocess, a narrow heater, such as a radio frequency single turn coil,surrounds a feed rod of polycrystalline material. The feed rod ispositioned above a single seed crystal that has a desiredcrystallographic orientation. The heater melts the feed rod material tocreate a liquid of material from which a resulting single crystal isgrown. A floating liquid zone is created, when the melted materialcontacts the single seed crystal. The feed rod polycrystalline materialis then continuously converted into single crystal. The term singlecrystal refers to a crystal that is in the form of a monocrystallinematerial.

[0006] A single crystal can be doped by solid or gaseous sources duringthe growth process. The gaseous dopants may be diluted into an inertcarrier gas and blown to the floating zone of the liquid silicon. Thesolid dopants may be incorporated into the feed rod. The dopants aredissolved into the liquid and then incorporated into the growingcrystal.

[0007] One disadvantage to the FZ process is that solid alloy material,which may be used to increase the performance of the resulting circuit,may not be able to be added without disturbing the liquid zone. Thus,the resulting single crystal may only be used in a limited number ofsemiconductor applications.

[0008] Another known bulk crystal growing process is the Bridgmanmethod. A closed tube configuration is employed in which a seed isplaced at one end of the tube in a liquid. The liquid is cooled to causethe liquid to solidify as a single crystal. The resulting single crystalhas a specific crystallographic orientation. One disadvantage to thistechnique is that unwanted stresses typically occur in the resultingsingle crystal.

[0009] Another known technique is the Czochralski (“CZ”) or crystalpulling method. A seed attached to a shaft is lowered into a liquidpool. The shaft is then simultaneously rotated and raised (pulled) in acontinuing sequence of precisely controlled steps. As the shaft israised, crystallization occurs where the liquid contacts the seed toform a single crystal. The resulting single crystal is then sliced in asawing process to produce individual wafers. Typically, a wafer may havea resulting diameter between 80-90% of the original diameter of thegrown single crystal. This is because the crystal may be ground, lapped,and polished before the wafers are formed. Using known CZ methods, asingle crystal having a length of about one meter and a diameter ofabout 80 mm may take a day or more to produce.

[0010] Known CZ methods are adequate for single crystal growth, when thesingle crystal is formed from one chemical element or a compound.However, the growth of solid solution alloy crystals requires specificprocessing steps that increase the complexity of the growing process. Inparticular, an alloy crystal requires substantially lower growth rates,and therefore more processing time, to maintain suitable crystal growth.This is necessary to prevent the alloy crystal from degrading into adendritic or polycrystalline form. For example, a processing step maytake about forty times longer for an alloy crystal compared to anelemental or compound single crystal. However, in known CZ methods, thetotal processing time is fixed. As a result, both the length anddiameter of a resulting alloy crystal is significantly limited.

SUMMARY OF THE INVENTION

[0011] The present inventors have discovered a method for growing largediameter solid solution alloy crystals. In accordance with one aspect ofthe invention, this is done by forming a solid solution alloy crystalhaving a diameter that varies with a diameter of a seed crystal. Theseed crystal is exposed to a liquid including a concentration of atleast one alloy element to dissolve a portion of the seed crystal. Thesolid solution alloy crystal may then be formed from the liquid. Becausethe seed crystal has a diameter at least the same as the resulting alloycrystal, the preferred method can be used to grow a larger diametersolid solution alloy crystal in a fixed total process time, as comparedto known systems. Alternatively, a large diameter solid solution alloycrystal can be formed in a shorter process time in accordance with thepreferred method.

[0012] Preferably, the seed crystal may be formed from a Czochralskiprocess, or may be a portion of a prior grown crystal. The solidsolution alloy crystal may be formed from a combination of germanium andsilicon.

[0013] In accordance with another aspect, the invention is directed to amethod for forming a semiconductor wafer that includes forming a solidsolution alloy crystal having a diameter that varies with a diameter ofa seed crystal. The seed crystal may be exposed to a liquid to dissolvea portion of the seed crystal, and the liquid may have a concentrationincluding at least one alloy element. The solid solution alloy crystalmay be formed from the liquid. A plurality of wafers may then be formedfrom the grown solid solution alloy crystal. An internal diameter sawmay be used to form the wafers from the solid solution alloy crystal.

[0014] These and other embodiments, aspects and advantages of theinvention will become more apparent in light of the following detaileddescription, including the accompanying drawings and appended claims.

[0015] The details of one or more embodiments of the invention are setforth in the accompanying drawings and the description below. Otherfeatures, objects, and advantages of the invention will be apparent fromthe description and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

[0016]FIG. 1 illustrates an exemplary Czochralski apparatus for forminga solid solution alloy crystal in accordance with a preferred method.

[0017] FIGS. 2A-2B illustrate a liquid formed in the Czochralskiapparatus of FIG. 1.

[0018] FIGS. 3A-3G illustrate various stages of the formation of a seedcrystal.

[0019] FIGS. 4A-4B illustrate various stages of the formation of a solidsolution alloy crystal in accordance with the preferred method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] A solid solution alloy crystal may be formed using a knownCzochralski process. A solid mixture including an alloy material isplaced in a crucible and exposed to suitable heat to melt the mixture toform a liquid. A seed is rotatably attached to an elongated member andimmersed partially into the liquid. Next, the seed is slowly pulled fromthe liquid while the seed is rotated at a predetermined speed, such as5-15 rpm. The liquid hardens around the seed to form the resulting alloycrystal as follows.

[0021] The diameter of the alloy crystal is initially expanded bydecreasing the pull rate for a predetermined processing time. Forexample, the pull rate may be decreased to less than 1 mm/hr. Next, thealloy crystal is grown at a constant pull rate to provide asubstantially constant diameter body. Finally, the alloy crystal isdetached from the liquid. The alloy crystal is then cooled for two hoursor more before removing the crystal and forming wafers.

[0022] The total process time to form an alloy crystal is limited bydissolution of the crucible and oxidation within the chamber in theabove Czochralski process. For example, the total process time may belimited to about three days. However, certain processing steps mayrequire about a day or more to complete. Thus, the diameter and lengthof the body of the resulting alloy crystal is limited. Additionally, apolycrystalline layer may form at the periphery of the seed crystal thatfurther limits the diameter and length of the alloy crystal.

[0023] Accordingly, the inventors have discovered that a solid solutionalloy crystal may be formed in a two stage growing process. The processforms a large diameter alloy crystal in a significantly reduced processtime, as compared to known methods. For example, the process time may bereduced by fifty percent. Alternatively, the alloy crystal may beexpanded to a larger diameter in the above fixed total process time, ascompared to known processes.

[0024] In the first stage, a seed crystal is formed having a diameterslightly greater than that of the resulting alloy crystal. The seedcrystal may be formed from a Czochralski process. Alternatively, theseed crystal may be formed from a prior grown crystal, for example, apreviously grown silicon crystal, solid solution alloy crystal, or anyother suitable crystal for semiconductor fabrication. In the secondstage, the seed crystal is exposed to a liquid containing aconcentration of silicon and an alloying material, until a portion ofthe seed crystal dissolves in the liquid. An alloy crystal is then grownhaving a suitable solid solution alloy crystal composition. The alloycrystal may be a silicon-germanium crystal, doped silicon-germaniumcrystal, a single crystal or other suitable solid solution alloy crystalfor semiconductor fabrication.

[0025]FIG. 1 shows an exemplary Czochralski (“CZ”) growing apparatus 1which may be employed to form a solid solution alloy crystal inaccordance with a preferred method. The CZ apparatus 1 includes acrucible 3 disposed in a chamber (not shown) having an elongatedcylindrical body 5 and an opening 6 at its upper end. The crucible 3 maybe formed from fused-silica, quartz, or other suitable material. Asusceptor 8 may be configured inside the chamber to support the crucible3. The susceptor 8 may be formed from, for example, graphite. A heater 4is positioned around a portion of susceptor 8 to supply suitable heat tothe interior of crucible 3 during operation. Heater 4 may be, forexample, an electrical resistance heater or inductive coil.

[0026] The CZ apparatus 1 also includes a rotating support member 10.The rotating member 10 engages the lower portion of the susceptor 8 torotate the susceptor 8 and the crucible 3 in a clockwise orcounterclockwise direction during the crystal growing process, asdescribed below. The rotating member 10 may be driven by, for example, amotor (not shown). Suitable speeds for the rotating member 10 may beabout 1-30 rpm.

[0027] A dispenser 40 may be positioned above the opening 6 of thecrucible 3 to supply a sufficient amount of powdered or granulatedmaterial 25 through opening 42 to the crucible 3. Material 25 may besilicon, germanium, or other alloying or doping material. More than onedispenser 40 may be used to supply individual materials to the interiorof crucible 3. The dispenser 40 may be formed from quartz or othersuitable material. The dispenser 40 may be rotated by, for example, amotor.

[0028] The CZ apparatus 1 also includes a pull shaft 65 disposed abovethe crucible 3 for holding a seed 67 made from, for example, silicon, onits lower end. The pull shaft 65 may concurrently rotate and verticallymove the seed 67 during the crystal growing process.

[0029] One or more gas lines 45 may positioned above the opening 6 ofthe crucible 3 to supply dopant or inert gases to the interior of thecrucible 3 at predetermined times. The gases may be employed to removereaction products from the chamber. Additionally, the gases may beemployed to maintain a suitable atmosphere within the chamber.

[0030]FIG. 2B illustrates a liquid 60 formed inside crucible 3 duringthe seed crystal growing process. Liquid 60 may fill about 30% ofcrucible 3. To form liquid 60, granulated silicon from dispenser 40 orsolid pieces of silicon 25 may be added to crucible 3 (FIG. 2A). Next,the heater 4 supplies heat to the exterior of the susceptor 8 to meltthe silicon to form the liquid 60.

[0031] As described above, the seed crystal may be formed from a portionof a previously grown crystal. The previously grown crystal may be asingle crystal or a previously grown solid solution alloy crystal. Theseed crystal may also be grown using the exemplary CZ apparatus 1 asfollows.

[0032]FIG. 3A illustrates a lower end of a seed 67 is partially immersedinto the liquid 60 by the pull shaft 65. Preferably, seed 67 has asmaller diameter than the seed crystal 92 (FIG. 3G). Next, the pullshaft 65 is pulled upward to form a short length crystal 2 from seed 67(FIG. 3B). The short length crystal 2 is formed to provide a startingpoint for the growth process of seed crystal 92. Pull shaft 65 mayrotate seed 67. Additionally, support member 10 may rotate susceptor 8and, thus, crucible 3 in a direction opposite of seed 67.

[0033]FIG. 3C illustrates a neck 80 formed from short length crystal 2.The neck 80 may have a diameter of about 4 mm. This process may occurduring the pulling of seed 67 from liquid 60 to remove dislocations fromthe crystal material. Preferably, the number of dislocations issubstantially zero. The term “dislocations” generally refers to a defectin the structure of the crystal. At this stage, the temperature ofliquid 60 is preferably greater than 1412□C, which is the melting pointof silicon. The seed 67 may rotate at a speed of 10 rpm, and the pullshaft 65 may lift seed 67 from liquid pool 60 at about 20 cm/hr.

[0034] As shown in FIG. 3D, a cap 90 may be formed below neck 80. Theseed 67 may be pulled upward by pull shaft 65 at a rate of about 25mm/hr, and the cap 90 may have a resulting diameter of about 65 mm. Ashoulder 85 is then formed contiguous with cap 90 by increasing the pullrate to about 12 cm/hr. (FIG. 3E)

[0035]FIG. 3F shows a body 88 is formed below shoulder 85. The body 88is grown to a diameter substantially the same as the resulting alloycrystal (FIG. 4B). The diameter of body 88 may be about 65 mm. The body88 may be truncated, as shown in FIG. 3G. The body 88 may have a largeror smaller diameter than shoulder 85. The body 88 of the resulting seedcrystal 92 may then be shaped or patterned by grinding or etching. Morethan one seed crystal 92 may be formed in a single batch crystal growingprocess.

[0036]FIG. 4A illustrates seed crystal 92 is exposed to a liquid 50.Liquid 50 is formed from a concentration of silicon 77 and an alloyingmaterial 78. Liquid pool 50 may be formed by adding specific amounts ofalloy material 78 from dispenser 40, and then exposing the material toheater 4, as described above. Alternatively, the alloy material 78 andmaterial 25 may be added to and heated in crucible 3 concurrently. Theconcentration of alloy material 78 may be greater than zero and lessthan 100 weight percent. For example, to grow a seed crystal having aforty weight percent concentration of germanium, the liquid 50 may havea concentration of about seventy weight percent germanium. Material 78may also include a doping material, for example, boron. Seed crystal 92contacts liquid 50 such that a sufficient amount of dissolution of body88 occurs.

[0037] As shown in FIG. 4B, a solid solution alloy crystal 75 may thenbe formed. The solid solution alloy crystal 75 may have a diametersubstantially the same or less than body 88. The alloy crystal 75 maythen be sliced into wafers by, for example, an internal diameter saw.

[0038] An example growth of an alloy crystal 75 in accordance with thepreferred method is as follows. Using inductive heating, a seven inchdiameter crucible is filled with preweighed silicon and germanium toproduce a total charge of 1500 grams. Under a helium flow sufficient tomaintain one atmosphere pressure in the growth chamber, the charge isheated to form a liquid. A seed crystal having a diameter of about 65 mmis exposed to the liquid to partially dissolve the seed crystal. Theseed crystal then is rotated at about 10 rpm in a clockwise direction,and the crucible is rotated in a counter clockwise direction at the samerate. Concurrently, the seed crystal is pulled upward from the liquid atabout 1 mm/hr. The temperature is also decreased at a rate sufficient tomaintain the alloy crystal diameter at least at 50 mm.

[0039] As described above, the diameter of the seed crystal 92 should beat least substantially the same diameter as that of the resulting alloycrystal 75. Further, growing the alloy crystal 75 from a previouslygrown seed crystal 92 may reduce the total process time. Alternatively,an alloy crystal 75 having an increased length may be grown for the sametotal process time (e.g., three days). Accordingly, larger diameteralloy crystals 75 may be grown than in known systems. The presentinventors have also discovered that minimizing the number ofdislocations in the seed crystal produces a solid solution alloy crystalwith a low dislocation density. Accordingly, the number of structuraldefects in the resulting wafer may be minimized. Additionally, the alloycrystal 75 may be grown in the same batch process as the seed crystal92. This means that less material may be used to grow large diameteralloy crystals 75.

[0040] The present disclosure has been described in terms of a number ofembodiments. The invention, however, is not limited to the embodimentsdepicted and described. For example, the crystal growth process may bevaried to achieve polycrystalline growth. Additionally, the diameter ofthe solid solution alloy crystal may be increased using a largerdiameter seed crystal.

What is claimed is:
 1. A method, comprising: providing a seed crystalhaving a first diameter at least as great as a solid solution alloycrystal to be formed; and forming the solid solution alloy crystal froma liquid, the liquid having a concentration including at least one alloyelement.
 2. The method of claim 1 , wherein the diameter of the solidsolution alloy crystal increases with an increase of the first diameter.3. The method of claim 1 , wherein the providing a seed crystal furthercomprises forming the seed crystal from a Czochralski process.
 4. Themethod of claim 1 , wherein the providing seed crystal further comprisesforming the seed crystal from a portion of a prior grown crystal.
 5. Themethod of claim 1 , wherein the solid solution alloy crystal is formedform a combination of germanium and silicon.
 6. The method of claim 1 ,wherein the seed crystal is formed from silicon.
 7. The method of claim1 , wherein the providing the seed crystal further comprises removing aplurality of dislocations from the seed crystal.
 8. The method of claim1 wherein the solid solution alloy crystal comprises a concentration ofalloy material less than 100 weight percent.
 9. The method of claim 1 ,wherein the solid solution alloy crystal is a single crystal.
 10. Themethod of claim 1 , wherein the providing the seed crystal furthercomprises doping the solid solution alloy crystal.
 11. The method ofclaim 1 , wherein the providing the seed crystal further comprises oneof patterning and shaping the seed crystal.
 12. The method of claim 1further comprising forming a plurality of wafers from the solid solutionalloy crystal.
 13. The method of claim 1 wherein the forming the solidsolution alloy crystal further comprises varying a diameter of the solidsolution alloy crystal with the first diameter of the seed crystal. 14.A method, comprising: providing a seed crystal having a first diameterat least as great as a solid solution alloy crystal to be formed;exposing the seed crystal to a liquid, the liquid having a concentrationincluding at least one alloy element; and forming the solid solutionalloy crystal from the liquid, the solid solution alloy crystal having adiameter that varies with the first diameter.
 15. The method of claim 14, wherein the providing a seed crystal further comprises forming theseed crystal from a Czochralski process.
 16. The method of claim 14 ,wherein the providing seed crystal further comprises forming the seedcrystal from a portion of a prior grown crystal.
 17. A method,comprising: providing a seed crystal having a first diameter at least asgreat as a solid solution alloy crystal to be formed, exposing the seedcrystal to a liquid, the liquid having a concentration including atleast one alloy element and silicon; and forming the solid solutionalloy crystal from the liquid, the solid solution alloy crystal having adiameter that varies with the first diameter.
 18. The method of claim 17, wherein the providing the seed crystal further comprises removing aplurality of dislocations from the seed crystal.
 19. The method of claim17 , wherein the solid solution alloy crystal comprises a concentrationof alloy material less than 100 weight percent.
 20. The method of claim17 , wherein the solid solution alloy crystal is a single crystal.